There is known a fuel supply device that separates, with a separator, raw fuel that contains components with different octane numbers, such as ethanol-blended gasoline, into high-octane fuel that contains a greater amount of components with high octane numbers than the raw fuel and low-octane fuel that contains a greater amount of components with low octane numbers than the raw fuel, and supplies the high-octane fuel and the low-octane fuel selectively to an internal combustion engine (for example, Patent Document 1). The fuel supply device includes a raw fuel tank for storing the raw fuel, a heater for heating the raw fuel, a separator for separating the heated raw fuel into the high-octane fuel and the low-octane fuel by pervaporation using a separation membrane, a cooler for cooling each of the separated fuels, and a high-octane fuel tank for storing the high-octane fuel. This fuel supply device can suppress knocking by increasing the ratio of the high-octane fuel injected into the combustion chamber when the internal combustion engine operates at a high compression ratio.